Spring RTS Engine

I am looking for a software or maybe even something like a app for a mobile phone which is capable of giving me a notification every 20 seconds, like a beep or a pop up or light flash.

It is of utmost importance!

I just look at the game timer to time quad/armor/mh

Perhaps your phone has a stopwatch, I don't know if you could set it to repeat or not.

making a soundfile of 20sec silence and with a beep at end would bei simple too -> audacity
then play on repeat

if you don't say your phone OS, there's not much help that can be done
there's iPhoneOS, Android, Symbian,Windows Mobile, etc ...